Combining Complex Models
Parenting is hierarchical .. so  depending on which parts of your model you want to control other parts, a object can have siblings and a parent, and its parent can have a parent of its own.. and so on. The 'family tree' is normally collapsed in the Outliner, reducing clutter. You can select multiple objects and parent them to the last selected (active) object simultaneously, using Ctrl P.

You could join all the objects you don't want to move independently, and make the turrets children of the joined object. (The turrets' children will become grandchildren of the joined object)

Or, create a heirarchy of parts. In it, you can make a link between any child and its parent 'rigid' with respect to translation, rotation, and/or scale by locking the child's own transformations using the padlocks in the 'Transform' panel of the Properties Region of the 3D View. This will mean only ancestors can move the  children in those ways. If you want to lock multiple children at once, select them, and hold Alt while locking.